The present invention relates to interprocessor communications systems and specifically to survivable local area networks.
Communications networks that link the data processors of geographically dispersed facilities are subject to failure from human error, facility destruction or a variety of natural and artificial calamities. Whether the transmission medium is through guided wires or through the air, such as by radio, optical or infrared waves, or microwaves, failure can occur randomly, often in unanticipated patterns.
An important component of a survivable distributed processing system is a communications network that can resist degradation due to node or link outages. Lccal area bus network technology has proven effective in interconnecting terminals and processors by providing flexible and high rate data service. Local bus networks employ a bus transmission structure to provide communications in a limited geographical area, such as an office building complex, air base, campus, or city. The bus permits many subscriber data processing nodes to be attached to a single physical link and affords two-way communications among them. All nodes can "hear" all signals, and any node can communicate directly with any other node. The bus also allows the passive attachment of nodes, so that failures in any node will not disrupt the network.
Traditional methods of improving the survivability of a bus network include hardening the bus so that faults are difficult to introduce; employing redundant busses and providing switching among them in the event of an outage, so that more faults are required to disrupt communications; or some combination of hardening and redundancy.
Hardening and redundancy is both expensive and a technically insufficient solution for making distributed processing system survivable. One fault in a bus can disrupt the bus communications and faults on all busses can disrupt the network. The task of enhancing the survivability of distributed processing systems has been alleviated to some degree by prior art techniques. The extent of these prior art techniques is given by the following patents: U.S. Pat. No. 3,162,827 issued to Border et al on Dec. 22, 1964, U.S. Pat. No. 3,275,749 issued to Kunihiro et al on Sept. 27, 1966, U.S. Pat. No. 4,075,440 issued to Laubengayer et al on Feb. 21, 1978, U.S. Pat. No. 4,254,496 issued to Munter on Mar. 3, 1981, U.S. Pat. No. 4,308,613 issued to Chasek on Dec. 29, 1981, and U.S. Pat. No. 4,380,061 issued to Mori et al on Apr. 12, 1983.
The patents of Munter, Chasek and Border et al disclose some developments beyond the traditional solution of redundancy. Border et al in addition to the costly redundant network of separate transmission facilities and separate communications lines, presents a primitive fuse assembly that disconnects branches when a short occurs. Munter's invention has a primary and secondary multiplexer bus with automatic switching between busses. Munter's device has limited improvements in system availability: a bus failure occuring in both the primary and secondary busses renders the system inoperable to all users. The Chasek invention provides a method of automatically bridging in-line failed stations emitting microwave transmissions or electromagnetic signals. While the automatic bridging feature is a development, Chasek's solution consists of redundancy in the form of a matrix of equi-distant in-line stations which is effective but expensive.
Laubengayer discloses a communication system which automatially reconfigures itself upon failure of detection of a carrier signal in one of the links. Switches are automatically actuated to connect the momentarily unconnected portion of the broken link to the link that is still intact. Communication is restored in the new link configuration. Laubengayer is an improvement over the Munter device. An entire bus or loop should not be disabled when a fault occurs: only the faulty area should be isolated. However, Laubengayer's device still has a limitation in that in the event of multiple loop failures, entire sections of the loop are cut off with the only design alternative being the duplication of loops in traditional redundancy.
Kunihian et al provides the most reliability and availability of all the prior art systems by a combination of design and redundancy. Each user is interconnected in a multiplex switching system with each transmision line. The problem with the approach is the excessive expense if one particular user is remote from one of the transmission lines. A better approach would not require each user to be connected to each line, but should have interconnections between lines that isolate only the faulty areas and provide cross communication between different transmission lines.
The goal of effective cross communications between transmission lines is best demonstrated by Mori et al where the terminal of each user is connected to a pair of transmission control units in two communication loops. The transmission control units bypass routes within loops where faults exists without disabling the entire loop. Unfortunately, the device of Mori et al is limited to a double loop transmission system.
In view of the foregoing discussion it is apparent that there currently exists a need for enhancing the availability of communication paths between geographically displaced data processing systems. The system should optimize the expense such that in addition to traditional redundancy of transmission lines, cross communication between separate lines should be available with only the faulty areas within the lines being isolated. The present invention is directed towards satisfying that need.